Dating of a large tool assemblage at the Cooper’s Ferry site (Idaho, USA) to ~15,785 cal yr B.P. extends the age of stemmed points in the Americas

The timing and character of the Pleistocene peopling of the Americas are measured by the discovery of unequivocal artifacts from well-dated contexts. We report the discovery of a well-dated artifact assemblage containing 14 stemmed projectile points from the Cooper’s Ferry site in western North America, dating to ~16,000 years ago. These stemmed points are several thousand years older than Clovis fluted points (~13,000 cal yr B.P.) and are ~2300 years older than stemmed points found previously at the site. These points date to the end of Marine Isotope Stage 2 when glaciers had closed off an interior land route into the Americas. This assemblage includes an array of stemmed projectile points that resemble pre-Jomon Late Upper Paleolithic tools from the northwestern Pacific Rim dating to ~20,000 to 19,000 years ago, leading us to hypothesize that some of the first technological traditions in the Americas may have originated in the region.

The carbonate aspect of the LUB3 deposit is pedogenic based on our observation of common, fine rhizolith features and hypocoatings in pores throughout the matrix, which indicates downward percolation and evaporation of soil water. We also see that carbonates accumulate on the undersides of bone fragments and artifacts, which is also indicative of pedogenic carbonate formation (48). If these carbonates were the product of phreatic processes all the artifacts in LUB3 and above would be coated and encased in with carbonate and they are not.
The slight reddish-pinkish colored loess seen in LU3 and LUB3 reflects the development of a cambic subsurface horizon, which includes in situ oxidation of parent materials but no significant evidence of clay translocation. The Rock Creek paleosol at Cooper's Ferry is like other loess paleosols described in the Columbia River Plateau region that include paleosol sequences with relatively thin cambic horizons overlying calcic horizons (e.g., 49,50). Darker, organic-rich A horizons are uncommon in Plateau loess paleosol sequences, lost either to surficial erosion prior to burial (as in the case of LUB3) or due to oxidation (i.e., decomposition) of the horizon's soil organics over time (49).

4-Methods for Identifying Possible Displaced Artifacts in Rodent Burrow Traces (Krotovina)
Burrowing rodents can displace artifacts vertically and horizontally because of their tunneling behavior in a site's sedimentary matrix. In many sites the identification of such displaced artifacts can be difficult because the color and texture of the burrow fills and those of their sedimentary matrix are very similar. Fortunately, in the early sedimentary deposits at Cooper's Ferry krotovina can be readily distinguished from the brown (10YR 5/3) loess of LU3/LUB3 by their much darker brown (10YR 4/3) and grayish brown (10YR 5/2) fill derived from mixing of the overlying depositional units.
Rodent burrow traces at Cooper's Ferry appear as cylindrical tunnels that crosscut through the site's different lithostratigraphic units and can be easily seen as patches and extended polygons on contrasting colored and textured sediments (fig. S14). The site's krotovina typically measure less than 10 cm in diameter, bear slope angles that range from 0-45 degrees, and can appear as individual tunnels or branching networks that extend through one or more lithostratigraphic units. To differentiate between in situ archaeological deposits and bioturbated deposits created by burrowing rodents, we developed a set of procedures at the start of the 2009 field season and applied them consistently during all years of excavation (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018). At the start of each excavation level, we identified krotovina by delineating their extents as they were seen in sediments to be excavated and assigning them a unique numerical identifier. During the process of level excavation, we isolated the sediments contained within each krotovina and first excavated non-krotovina sediments, leaving the areas with infilled burrows to stand as pedestals of sediment. Upon reaching the end of the excavation level, the pedestaled krotovina were photographed and their spatial extents recorded with a total station. Finally, the sediments in each krotovina were excavated and screened separately with their archaeological contents bagged and cataloged. By identifying, isolating, recording, and excavating each rodent burrow trace encountered, we were able to separate objects that had been displaced in the site stratigraphy due to bioturbation. We did not use the total station to spatially record items that were found within krotovina to avoid mixing up ex situ and in situ finds. This process worked well in helping us distinguish possible displaced artifacts within the loess deposits of LU3/LUB3 but was less useful in identifying possible burrowing within the archaeological pit fills wholly contained within the loess. This was because the dark archaeological deposits within the pits, which allowed us to readily distinguish the pit dimensions and deposits from the surrounding loess in the same way we identified krotovina, were not visibly different from any potential rodent burrows within them. Fortunately, the pedogenic carbonate deposits associated with the formation of Rock Creek soil proved to be a useful tool in recognizing the in situ provenience of artifacts within the pit fills.
As with the other artifacts from LUB3, lithic artifacts >1 cm diameter from within the pit fills have extensive artifact coatings resulting from the formation of the Rock Creek soil after the artifacts were deposited. The presence of this carbonate coating confirms these artifacts were not displaced from above by rodent burrowing. On lithic artifacts ≤1 cm diameter, however, carbonate pendants are found differentially on some but not all the smaller lithic artifacts. Carbonates tend to be present on smaller items made of fine grained volcanics but are not consistently present on CCS or obsidian. This may be due to the texture of these toolstone types combined with their small size, with coarser FGV more readily hosting carbonate pendant development than smoother CCS and obsidian. Hence, it is not always possible to confirm the depositional integrity of the very smallest lithic items from within the LUB3 pit fills.
The story is more straightforward for bones from both the pit fills and the LUB3 sediments in that they are grayer in color, more physically weathered, and have thicker carbonate coatings (fig. S3) than items in the younger, overlying LUB4 and F99 deposits. These are readily distinguished from a few small bone fragments from within the pit fills which do not share these characteristics, and which resemble bones from overlying depositional units.

5-Discussion of Archaeological Features
During excavation of LUB3, our team identified deposits that did not match the sedimentological character of known lithostratigraphic units, and which might signal the presence of a different lithostratigraphic or pedostratigraphic unit, an infilled rodent burrow trace, or patterns of sediment and soil alteration resulting from various cultural activities during past occupation of the site. These cultural activities include a range of site formation actions that impart observable characteristics of modification to the site's stratigraphic units, such as digging and refilling pits, heating with fire, and discarding materials that do not match site sediments (e.g., piling rocks, spreading different colored and textured sediments across a surface bearing different geological properties). These processes of site formation are more complex than the simple act of discarding artifacts or faunal materials on an ancient surface. Instead, these processes create definable threedimensional shapes, which are known as archaeological features. We identified and recorded three archaeological features that were dug into an ancient surface that was present during the original deposition of LUB3 sediments. These three features were interpreted as cultural pits excavated and backfilled in the Pleistocene by the site's human inhabitants. This interpretation is based on the observation that LUB3 pit features are consistent in size, shape, and archaeological content with other pit features that have been described previously from Area A (1,12,41) and from Area B (13,42). The artifact and faunal content of these previously undiscovered pit features from LUB3 is consistent with other reported cultural pits and as such, they are generally interpreted to have probably been used to cache equipment and dispose of camp refuse. Intentional caching of points can be inferred from the clustered spatial pattern of stemmed points within F78 and F108 ( fig. S4). Broken points may have been included in these caches by the site's occupants with an intention to repair them later, or their internment might have had some other unknown symbolic reason.
Because the pit features were too cylindrical, too large, and too uniform in shape, contain relatively high concentrations of artifacts and faunal materials, and can retain piles of cobbles at their upper limits that suggest cairn construction, we do not think these pit features were caused by large burrowing animals (e.g., coyote, badger) or because of an overturned tree (i.e., "tree throw"). Moreover, their artifact and faunal material content, including concentrations of projectile points, is entirely consistent with other published cultural pit features found previously at the Cooper's Ferry site (1,12,13,41,42).
Feature 108 was a cylindrical pit measuring ~90 cm in diameter, roughly oriented along a southwest to northeast axis. Excavation initially identified this feature from its circular pattern of darker colored sediments that contrasted with the surrounding LUB3 deposits ( fig. S7). Excavators mapped 81 artifacts and fragments of faunal materials in situ, including seven stemmed projectile points ( Fig. 4 and fig. S10 and S11). Two animal bone fragments excavated in situ returned two AMS ages of 13,147±55 yr B.P. (15,672 cal yr B.P.) and 13,146±59 yr B.P. (15,859-15,666 cal yr B.P.).
Feature 151 was a cylindrical pit measuring ~75 cm long and 60 cm wide, roughly oriented along a northwest to southeast axis. Excavation initially identified this feature from a pile of pebbly loamy sand at its surface, underlain by a circular pattern of darker colored sediments that contrasted with the surrounding LUB3 deposits ( fig. S9). Excavators mapped eight pieces of debitage and 16 fragments of animal bone in situ, within the pit (fig. S11). Three animal bone fragments returned AMS ages of 13,260±240 yr B.P. (16,240 cal yr B.P.), 13,226±52 yr B.P. (15,970-15,790 cal yr B.P.), and 13,091±48 yr B.P. (15,617 cal yr B.P.).

6-Near Infrared Analysis
We used near-infrared (NIR) spectroscopy to non-destructively prescreen bone samples excavated from LUB3, F78, F108, and F151 for collagen preservation following methods described by Sponheimer et al. (51). The frequency and amplitude of collagen in the NIR range reflect the type of vibration, concentration, and absorptivity of hydrogen bonds in amino acids and other hydrogen-rich molecules (C-H, N-H, O-H, S-H) (52). Since near-infrared light penetrates approximately 8.5 mm into biological tissues (53,54), NIR is an ideal technique to analyze the preservation of collagen in the bone because the outer surface of bone is vulnerable to diagenesis (55)(56)(57).
A Labspec 4 spectrometer (Malvern Panalytical) equipped with a fiber-optic probe was used to collect the NIR spectra (each spectrum an average of 50 scans) from each bone sample. Each spectrum takes a few seconds to collect, and we prescreened 776 samples over three days at Oregon State Univerisity. We used principal component analysis, partial least squares regression, and visual inspection of spectra from specimens of known collagen yields to characterize the unknown specimens (51). These comparative data were then used to make decisions about which bone samples were likely to contain enough collagen for successful AMS radiocarbon dating at the Oxford Radiocarbon Accelerator Unit laboratory (fig. S15).

7-Radiocarbon Dating
Fragmentary bone samples from medium to large animals were submitted for AMS analysis at the Oxford Radiocarbon Accelerator Unit. Methods for sample pretreatment and radiocarbon measurement follow a previous publication on the site (1) where these are described in further detail. Calibration and Bayesian modelling were undertaken using the IntCal20 calibration curve (31) and the OxCal 4.4 software programme (58). Chronometric data, including radiocarbon and luminescence dates (with a measurement date of 2012) for Area A (Table S4) and Area B (Table  1), were included in a multi-phase Bayesian model using stratigraphic information as a prior. The 'General' outlier model was used to identify and downweight outlying dates, with each assigned a prior outlier probability of 5% (59). All age estimates here are given at 95.4% credible/confidence intervals (CI) and rounded to 5 years (default resolution).

8-Supplemental OSL Dating
During the early excavation phases of Area B, we collected six quartz optically stimulated luminescence (OSL) samples to serve as rough guides for the approximate ages of the Area B deposits. While the resulting 2σ OSL age ranges of 2000-3000 years are not precise enough to distinguish individual depositional events, they do provide broad support for the more detailed radiocarbon-based chronology.

Sample Collection, Preparation and Measurement
The OSL samples were collected by driving steel tubes (20 cm long, 5 cm diameter) into cleaned vertical sections in Area B. Each tube was sealed in a black plastic and taped shut to avoid light exposure and moisture loss. In the OSL laboratory at Lanzhou University the sediments at each end of the cylinders were removed and used for water content and dose rate measurements. All six samples were measured using a standard single aliquot regenerative-dose (SAR) dating protocol on coarse-grained (90-125 µm) quartz grains (60). The purity of the quartz was checked by using an infrared (IR) depletion ratio test (61). A preheat plateau test was applied to one sample to determine the SAR measurement conditions. The result shows that the SAR protocol with a preheat temperature of 240 ºC and a second preheat of 200 ºC is suitable for equivalent dose (De) determination (fig. S16). All the samples were measured using a large aliquot size of 4-5 mm to retrieve detected signals. The natural and regeneration dose OSL signal decreased rapidly during the first second of stimulation, indicating that the OSL signal is dominated by a fast component ( fig. S17a). The growth curve was readily fitted using a single saturation exponential (fig. S17b). The average recycling ratio of all aliquots for six samples are 0.98±0.01 (fig. S17c), and the average recuperation ration of natural signal of almost aliquots for six samples are less than 5%, except few aliquots varying between 5-13% (fig. S17d). The Desoverdispersions for all aliquots of the samples are less than 20%. Together these all indicate the quartz OSL dating measurements are reliable. The De values and dose rate of all quartz samples were calculated using LDAC program v1.0 (62).

Water Content Estimation
Estimation of the average water content of sediment samples over their depositional history is an important unknown variable in determining OSL age estimates. The problem is particularly fraught at Cooper's Ferry due to its unique depositional history. In the loess regions of western China and elsewhere measured water content usually ranges from ~5-15% and an estimated water content of 10% is often used in calculations of age estimates (63). The measured water content of all the Cooper's Ferry samples was also below 10%, but the average water content over the life of the samples was likely much higher. Sometime after the deposition of the loess the nearby Salmon River began to aggrade as the result of a major neotectonic event in the lower Salmon River Canyon (64), with the level of the Salmon River eventually exceeding the elevation of the Cooper's Ferry and saturating the loess. For example, as shown in Fig. 1, a distributary channel of Rock Creek, which grades to the Salmon River, overlie virtually the entire intact Area B depositional sequence. Sometime after ~2000 cal BP, the Salmon River cut through the downstream obstruction, the river incised to near its present elevation, groundwater levels dropped, and the loess dried out (64). As a result of this complex depositional history, the average water content of the OSL samples is difficult to estimate and we have therefore calculated multiple age estimates using assumed water content levels of 10±5%, 20±5%, 30±5% as shown in Tables S6-S8, respectively.

High Water Content Age Estimates
The calculated age estimates using water content estimates of 30% are generally consistent with the 14 C dates from the same stratigraphic units (fig. S18 and Table S9). The exception is the sample from LUB4. LUB4 is a brief alluvial episode that post-dates the erosional event at the top of LUB3 and which likely reworked some of these earlier sediments. Alluvial sediments may produce inaccurate OSL ages, usually aberrant older age estimates, due to poor resetting or partial bleaching of some of the sediments as they are transported and redeposited (65).

Summary
Six OSL age estimates from Area B generally support the radiocarbon-based chronostratigraphy for Area B. However, a complex depositional history makes it difficult to adequately assess the average water content of the samples used in the age estimate calculations.

9-Bayesian Modeling
For Area B, sensitivity testing was conducted to test the impact of OSL ages on the chronology described in the main text (henceforth Model "A", which only includes radiocarbon data; Fig. 3).
Each alternate model used OSL measurements calculated using different water content estimates (Tables S6-S8). Model "B" included OSL ages with a water content of 10±5% (fig. S20), Model "C" included the OSL dates calculated with a water content of 20±5% ( fig. S21), whilst Model "D" used dates calculated with a 30±5% water content ( fig. S22). In fig. S23, we plot the difference in age between the start boundaries of Model A compared with Models B, C and D. The results are statistically identical, suggesting that differences in water content estimates within OSL age calculations do not have a significant impact on our Bayesian chronology for Area B. This is unsurprising since, except for 73-15-OSL-Lu2-2, OSL ages at the base of the sequence (LUB2-3) are generally identified as major outliers in the alternate models and downweighted accordingly (hence the comparable start estimates for LUB3 as seen in fig. S23). Moreover, the uncertainty in the water content values captures the bulk of variation in the resulting OSL ages at two sigma. Further work is required to better understand the offsets between radiocarbon and OSL ages.

Methods for Studying Projectile Points
The 13 projectile points found in LUB3, F78, and F108 were examined under low magnification (10-20x) and measured with digital calipers. A 3D digital model of each point was also created with a David SL-3 structured light system. The resulting digital XYZ point cloud models were processed and their geometric morphometric attributes were characterized using the GLiMR approach (43,66). Calcium carbonate adheres to blade margin. RN 43733;N 58.96,E 131.655,EL 410.83;Fig. 4). Bifacially flaked projectile point with stemmed haft. Convex haft margins contract below moderate to slight shoulders to a narrow rounded base with minor fracturing on one side. Haft margin shows edge grinding. Maximum length = 57.9 mm. Maximum width = 18.3 mm. Maximum thickness = 6.1 mm. Manufactured from white and tan CCS material. Plano-convex in cross section. Flaking performed in roughly collateral pattern with flakes removed at right angles to the long axis of the blade and haft. Partial ventral surface of original macroflake remains on one side. Edge damage apparent on one blade margin. Blade resharpening is minor and limited to distal blade portion. Calcium carbonate adheres to blade margin. RN 44347;N 59.387,E 131.55,EL 410.953;Fig. 4). Bifacially flaked projectile point with stemmed haft. Convex haft margins contract below moderate to slight shoulders to a narrow rounded base with minor fracturing on one side. Haft margin shows edge grinding. Maximum length = 50.3 mm. Maximum width = 18.2 mm. Maximum thickness = 5.1 mm. Manufactured from brown CCS material. Biconvex in cross section. Flaking performed in roughly collateral pattern with flakes removed at various angles. Blade resharpening is apparent with minor retouch along margins and prominent ears at haft-blade transition. Calcium carbonate adheres to one margin. RN 44612;N 59.016,E 131.571,EL 410.859

Preliminary views on 3D Geometric Morphometry of Early Projectile Points at Cooper's Ferry
Preliminary evaluation of the 3D geometric morphometric attributes for these points reveals some important patterns. Haft reentrant (i.e., the negative space that lies between the convex hull and the haft margin; fig. S24) size and shape is relatively small in the older F108, F78, LUB3, and Area A points, and increases in size over time in points from Pit Feature A2 (PFA2) and Pit Feature P1 (PFP1) (figs. S25-S27). This pattern shows that there are clear differences in the haft design of stemmed points found in the lower and upper portion of the LU3/LUB3 deposit and signals what may be a progressive evolutionary sequence in the development of hafting design at Cooper's Ferry. More work is needed to fully assess morphometric differences among stemmed points at the Cooper's Ferry site.

Other Stone Tools
One small fragment of a biface made on CCS material was found in F78. A burin spall made on CCS material showing the creation of a burin tool was found in F78. A small fragment of a unidirectional flake core made on CCS material was found in F78.

Debitage Analysis
We employed a lithic attribute analysis system that is grounded in experimental studies of lithic reduction sequences and their corresponding products to provide a preliminary analysis of all provenienced debitage (67)(68)(69)(70). Visual observations of debitage were made with a Leica zoom binocular microscope at magnification ranging from 3.5X to 22.5X and lithic attributes were recorded.
Debitage from LUB3 (n = 10) is mostly made from cryptocrystalline silicate (CCS) rock and analysis indicates early and late stage biface reduction via percussion. One flake shows early stage core reduction. Four pieces of fine grained volcanic (FGV) debitage show percussion reduction of early stage bifaces.
Feature 78 debitage (n = 250) shows primarily early and late biface reduction of CCS material through percussion and pressure methods, followed by early and late core reduction of CCS. Less common are FGV flakes showing early biface reduction and early core production, mainly through percussive reduction. One small obsidian pressure flake was found in situ within F78. Xray fluorescence analysis produced indeterminate results due to the object's small size.
Feature 108 debitage (n = 53) is dominated by CCS materials made during early and late stage biface reduction. One FGV flake shows early biface reduction, and three others show indeterminant percussion reduction.
Feature 151 contained eight in situ pieces of debitage. Analysis of these CCS and FGV pieces show a range of percussion and pressure techniques applied to make bifaces at early and late stages of production.
Artifact Totals from LUB3, F78, F108, F151, and LU3 Frequencies of artifacts found in LUB3 and its early pit features, and in LU3 (1), are reported in Table S2. Two Levallois-like cores, previously reported by Davis and Willis (71), were excavated in situ from LU3 in Area A and are included here in the artifact totals.

Examination of Faunal Materials
Authors L.G.D. and C.W.E. examined the bone fragments found in situ and from screened sediments within LUB3 and features 78, 108, and 151 to identify any potential human skeletal remains and to evaluate whether specimens could be assigned to taxonomic groups. No definitive human remains were identified among the bone fragments recovered from Area B. L.G.D. reviewed all bone submitted for radiocarbon analyses and did not observe any specimens bearing cortical thicknesses or morphological and anatomical attributes consistent with human bone. The bone specimens from LUB3 and its inclusive cultural features were very fragmentary and generally lack clear anatomical features and could be only identified to the class mammalia based on their size and cortical thickness. The excavated bone fragments most likely represent different small, medium, and large bodied mammals. Because of this situation, we hope to use instrumental analyses (e.g., ZooMS) to provide faunal identifications of the bone fragments. Knowing more about the animal taxa present in these pit features could help clarify paleoecological conditions, economic patterns of site occupants, and help to interpret the function of the pit feature. Frequencies of faunal materials found in LUB3 and its early pit features, and in LU3 (1), are reported in Table S3.           Projectile points 73-54105 was found above F108 in the upper portion of LUB3 and point 73-49277 was found in LUB3 sediments but deeper than the tops of the pit features. Black lines around plotted items show approximate shape of pits, as revealed during excavation.    Figure S15. NIR bands reflect collagen content. A) Absorbance spectra (second derivative; 31 points smoothing) from known archaeological specimens yielding 0.0% collagen (maroon), 5.5% collagen (turquoise), and 14.2% collagen (blue), and two specimens from Cooper's Ferry-one sampled (fuchsia) and one unsampled (red). The unsampled Cooper's Ferry specimen showed strong similarities with archaeological specimens lacking collagen in bands/regions reflecting collagen content (arrows at 1690 nm, 1730 nm, 2050 nm, 2180 nm, and 2280 nm) and was therefore not sampled for radiocarbon dating. The sampled Cooper's Ferry specimen, in contrast, retained clear evidence of collagen preservation-possibly greater than 5%--and was therefore an ideal candidate for radiocarbon dating. B) PCA scores plot (PC1 and PC2) of the collagenrelated regions of the NIR absorbance spectra (1671-1751 nm, 2030-2060 nm, 2153-2200 nm, 2250-2303 nm; second derivative; 31 points smoothing) of 32 known larger whole bone samples from archaeological sites and 776 other smaller Cooper's Ferry samples (gray triangles). Among the known archaeological samples, specimens with more than 3% collagen (blue circles) separate from low collagen specimens (<1%; red squares). Along PC1 (89% of variation), Cooper's Ferry samples typically cluster with specimens containing <1% collagen. Only 12.2% of the Cooper's Ferry samples were deemed good candidates for AMS radiocarbon dating (those within the blue cluster). Only 4.6% of the samples appeared sufficiently well preserved to make successful collagen extraction almost certain (those nearer the blue group's centroid).     S19. Probability density functions for the difference ("D") between the start and end boundaries of LU3, as estimated by Davis et al. (1) and in this analysis. These results suggest that there is 95.4% chance that the modelled outputs have no significant difference. Fig. S20. Probability density function for the difference ("D") between the start of Area A LU3 ("O"; from model in Fig. 5) and Area B LUB3 ("N", from model in Fig. 6). These results suggest that there is 95.4% chance that the modelled outputs have no significant difference.      (66). Reentrants are the negative spaces between the convex hull (i.e., the smallest convex polygon that contains all the artifact scan's XY spatial points) and the artifact's outline (72,73). In the case of flaked stone tools, reentrants commonly represent areas of material removal along margins (e.g., serrations, denticulations, stemmed hafts, and notches).  Fig. 4), Pit Feature A2 (42), and Pit Feature P1 (43). The left-side reentrant has been mirrored about the long axis of the point to allow better comparison with the right-side reentrant. Therefore, there are two similarly oriented reentrants shown for each projectile point. Reentrants are organized from oldest (PF108 at left) to youngest (PFP1 at right) assemblages.  (1)), LUB3 (73-54105; Fig. 4), Pit Feature A2 (PFA2) (42), and Pit Feature P1 (PFP1) (43). Reentrants are organized from oldest (PF108 at left) to youngest (PFP1 at right) assemblages.  Table S1. Stratigraphic description of deposits exposed along the a-a' transect, Area B.
LUB14: Brown (10YR 4/3; 10YR 5/3 dry) massive, fine pebbly loamy sand. This deposit is contained in intrusive channel features that eroded into underlying LUB12, LUB11a, LUB5 sediments. The lower boundary of the unit is unconformable with a sharp and irregular form.
LUB12: Pale brown (10YR 6/3 moist; 10YR 7/2 dry), massive, moderately well-sorted sand without visible structures. Mica flakes and biotite accompany a large percentage of quartzitic and plagioclase sands. The lower boundary of this unit is sharp and smooth.
LUB11a: Brown (10YR 5/4 moist; 10YR 7/1 dry), massive, moderately well-sorted sand without visible structures. Mica flakes and biotite accompany a large percentage of quartzitic and plagioclase sands. The lower boundary of this unit is sharp and smooth.
LUB3: (S1) Brown (10YR 5/3; 10YR 6/3 dry), massive, fine sandy loam with friable to firm consistence, bounded below by a clear conformable boundary. Calcium carbonate horizon is present as fine filaments, hypocoatings, and forms pendants on the undersides of artifacts and bone fragments throughout the deposit. This unit bears the Rock Creek Paleosol that shows erosional truncation at its upper surface shared with LUB4.
LUB2: Yellowish brown (10YR 5/4 moist; 10YR 8/1 dry), massive, moderately well-sorted sand. Calcium carbonate appears as dispersed hypocoatings and fine filaments throughout the sediment matrix. This lowermost sand has a sharp irregular basal boundary that appears to be conformable.
LUB1: Subrounded to subangular basalt clasts of fine pebble to fine boulder size with no apparent bedding structure in a relatively poorly sorted, clast-supported matrix. Carbonates coat the undersides of clasts in some areas. Lower boundary was not observed. Table S2. Quantities of artifacts in LUB3 and early pit features in Area B, and LU3 in Area A. FCR = fire cracked rock. MF = modified flake. PPT = projectile point.    Table S4. AMS radiocarbon ages from Area A. RN is the reading number. The percent collagen is the yield of extracted collagen as a function of the starting weight of bone samples. C:N is the atomic weight ratio of carbon to nitrogen. %C is the percentage of carbon in the combusted sample. Stable isotope ratios of C and N are expressed in per mil (‰) relative to Vienna Pee Dee belemnite and ambient inhalable reservoir. The calibrations were done using the OxCal 4.3 software (59) and the IntCal20 calibration curve (58). Missing chronometric data (*) are due to a lack in reporting or measurement on behalf of the laboratories. CI, confidence interval; -, not determined.  Table S5. Radiocarbon ages from living freshwater river mussels. pMC is the percentage of modern carbon in the combusted sample. Stable isotope ratios of C are expressed in per mil (‰) as measured by accelerator mass spectrometry on the reduced graphite target material.